1. Field of the Invention
The present invention relates to a disc shaped, zinc oxide-based varistor for high voltage or high current applications, and more particularly relates to improvements in the physical stability thereof.
2. Description of the Prior Art
A zinc-oxide-based varistor, as typically manufactured, comprises a sintered disc of zinc oxide and additives, the disc having a pair of electrodes on opposite faces. It is also typical for the electrode on the face to only 22 extend part way to the rim of the disc, in order to avoid arcing, also called fringing current or flash over. See, for example, U.S. Pat. Nos. 4,460,497 to Tapan K. Gupta et al., 4,451,815 to Eugene Sakshaug et Ano., and 4,450,426. In addition, the rim is usually protected from the elements or some aspect of the manufacturing process by an electric insulator. See, for example U.S. Pat. Nos. 4,371,860 to John E. May et Ano., and 3,138,686 to Steven P. Mitoff et Ano.
The zinc oxide varistor exhibits a non-linear current-voltage relationship, thought to be in the form I=C.times.V.sup.a, where "a" is greater than 1. In other words, it acts as an insulator for low voltages, and as a conductor for high voltages. It thereby provides overvoltage protection or acts as a voltage stabilizer, surge absorber or arrester, and may be subjected to current surges.
Because of Joule heating, the interior of the disc may reach a high temperature while the rim remains close to ambient temperature. The situation is exacerbated by the anti-arcing design in which the margin of the face is left bare. The electric field drops suddenly near the rim, as does the temperature, resulting in a thermal shock condition. This may result in physical cracking on the rim, and substantial damage to the device.
The following patents are hereby referenced as being typical of known prior art in so far as they disclose means for discouraging arcing and thereby minimize this problem and in which it appears that the electrode extends at least as far as the rim:
______________________________________ U.S. Pat. No. Inventor ______________________________________ 4,692,735 Moritaka Shogi et al. 4,423,404 Gary L. Goedde et Ano 4,272,411 Theodore O. Sokoly et Ano 3,905,006 IGA Atsushi, et al ______________________________________
In Shogi, Goedde and U.S. Pat. No. 3,905,006, the process for manufacturing and attaching the insulator is complicated and may involve temperatures over 500 degrees C., which may damage the varistor.
It is an object of this invention to provide an improved zinc oxide varistor disc which has more uniform Joule heating without causing arcing between the face electrodes, and which is thus less likely to crack on the rim and thus would provide more stability than prior art zinc varistor discs.
It is a further object of this invention to provide a system based on infrared thermal measurements to predict the energy handling capacity of a particular varistor disc.